These monsters are needed for the Blood of the Brood quest. But the quest update is uncommon and you may have to wait for respawn to get it. Varying information exists on the internet about their location. These monsters spawn in the second half of the ring event at the top of the dungeon, to the West. They spawn with the small green dragon, Palace Adjutant Drakknair (above The Nest of the Overseers), after the area is completely clear.

More also reside on the 5th floor on the other side of the palace. To get there, you go up to the third floor, and go to the far side(west) of the third floor there is a second climbable wall (in the forge). Go up this wall and you will be in "The Nest of the Skylords". Take the lift to the top of that nest (Erudition Aviary) and you will find the Bloodscale Dreadknights.